Recent advances in silicon-based image detectors have opened up possibilities for photography in a very wide waveband ranging from the deep UV (<200 nm) all the way to about 1100 nm in the near infrared. For many practical photographic purposes the UV portion of this waveband is limited to about 315 nm by atmospheric absorption of sunlight. The spectrum from 315 nm to 400 nm is commonly called the UVA spectrum. So, a very useful photographic spectrum will range from the onset of significant atmospheric transparency at about 315 nm all the way to the limit of silicon-based detector technology at about 1100 nm. A lens/camera system capable of high quality imagery over the 315 nm-1100 nm waveband would have a broad range of applications, including forensics, crime scene documentation, art conservation, forgery detection, medicine, scientific research, and fine art photography.
Of course, there are many imaging objectives in the prior art that are corrected for small portions of the 315 nm-1100 nm spectrum, but what is both lacking and needed are objectives offering both good transparency and good optical correction over the entire 315 nm-1100 nm spectrum.
In the prior art there are a number of examples of optical designs suited for a modest waveband in the near-UV. Typically these designs have ordinary two-wavelength achromatism, and their performance begins to degrade in the green or yellow portion of the visible spectrum. Examples include U.S. Pat. Nos. 2,663,222; 3,348,896; 3,517,979; 3,652,151; 4,050,778; and 4,387,970. These designs have unacceptable absorption at wavelengths below about 350 nm, and they also have extremely poor correction in the visible red and near infrared.
The prior art also includes examples of optical designs that use fused silica and CaF2 to achieve true apochromatic correction from the deep UV at about 250 nm through the visible waveband at about 650 nm. Examples include U.S. Pat. Nos. 3,486,805; 3,490,825; 5,103,341; 5,305,138; 5,699,202; 5,754,345; 5,798,874; and 5,914,823. The advantage of this type of design is that the optical transmission is extremely good, even at very short wavelengths down to about 190 nm. Unfortunately, the dispersions of fused silica and CaF2 become very similar for wavelengths longer than about 650 nm, so it is impossible to adequately correct chromatic aberration into the near infrared.
Also of interest are optical designs that are well corrected from the visible into the near infrared or even into the mid-infrared, but are not suited to use in the ultraviolet either because of a lack of optical correction or a lack of transparency or both. Examples include U.S. Pat. Nos. 4,206,972; 4,681,407; 4,704,011; 4,712,886; 4,832,472; 4,929,071 and 6,208,459.
Finally, there are optical designs that are corrected from the near-UV through the near-IR, but are not suited for use for the shorter wavelengths in the UVA waveband ranging from 315 nm up to about 350 nm. The reason for this shortcoming is either due to a lack of optical correction or a lack of transparency, or both. Examples include U.S. Pat. Nos. 4,702,569; 4,704,008; 4,761,064; 4,762,404; 4,765,727; 4,790,637; 5,000,548; 5,020,889; 5,204,782; 5,210,646 and 5,305,150. Even though some of the examples disclosed in these patents are color corrected at three, four, and even five wavelengths, none have adequate color correction below about 350 nm and many have extremely high absorption in the UV due to the glasses they use.
Accordingly, there is a need for an optical system that has both good optical correction and good optical transparency over the 315 nm through 1100 nm waveband.